1. Field of the Invention
The present invention generally relates to second-order non-linear optical (NLO) materials and in particular to ionic self-assembled multilayer (ISAM) films, and more particularly to fabrication techniques for these films.
2. Background Description
I. Background Summary
Second-order non-linear optical materials are at the heart of telecommunications devices such as electro-optic modulators and optical switches, and in lasers such as high power green and blue solid state lasers and optical parametric amplifiers.
Conventional NLO materials generally consist of inorganic crystals such as KTP, LiTaO3 and LiNbO3. While they are quite efficient, high quality crystals of sufficient size are expensive and difficult to manufacture. Organic NLO materials provide an alternative with the potential to provide high non-linear susceptibilities in an economical fabrication process. NLO materials based on ionic self-assembled multilayer (ISAM) films are particularly promising because of the ease of tailoring noncentrosymmetric structures and the long-term stability. These films are made by alternately immersing a substrate in two solutions, containing a polycation and a polyanion, respectively. If the substrate initially carries negative surface charges, dipping in the polycation solution will result in a nanoscale polymer layer self-assembled on the substrate, yielding a positively charged substrate. Subsequent dipping in the polyanion solution results in a second layer of the polyanion formed on top of the first layer. The process can be repeated as many times as desired, building up films to arbitrary thickness with nanoscale precision. ISAM films may have substantial second-order non-linear susceptibility (χ(2)) values, comparable to that of lithium niobate. Various methods have been suggested to improve the effective χ(2) of these films by modifying their composition.
Nanoparticles made from noble metals such as silver or gold have recently attracted considerable attentions due to their unusual optical properties which enable light to be controlled in unique new ways. The interaction of light with the free electrons in such particles gives rise to collective oscillations of the conduction electrons at optical frequencies, known as localized surface plasmon resonances (LSPRs). When excited in this fashion, the particles act as nanoscale antennas, concentrating the electromagnetic (E-M) field into very small volumes adjacent to the particles. Exceptionally large enhancements in E-M intensity can be obtained this way, by as much as a factor of 104 in individual particles, and 105 in dimers (i.e. two closely spaced particles linked together). Enhancements as large as 107 have been reported in nanoparticle clusters, enabling, for example, Raman spectroscopy of single molecules.
Other than Raman spectroscopy, this phenomenon has found applications in optical third-harmonic generation, as well as in second-harmonic generation, which is the subject of this disclosure. Nahata et al., Optics Letters 2003, 28, (6), 423-425), demonstrated an ˜104 fold increase in the efficiency of optical second-harmonic generation from concentric silver ring structures centered around a 200-nm aperture (bull's-eye structure) compared to a silver film with an unadorned aperture. Podlipensky et al., Optics Letters 2003, 28, (9), 716-718, observed second harmonic generation (SHG) enhancement from ellipsoidal silver nanoparticles in a glass matrix grown by means of Ag+—Na+ ion exchange. More recently, Moran et al., J. Phys. Chem. B 2005, 109, (10), 4501-4506, have demonstrated second harmonic excitation spectroscopy of silver nanoparticle arrays, which were fabricated by nanosphere lithography. In that work, a weak out-of-plane LSPR mode was made to overlap with the second harmonic of the incident laser, so that the resulting SHG emission signal became proportional to the plasmon enhancement.
All of these studies used the nanoparticles in a dual role—as concentrators of the electromagnetic field, and as the NLO media. Since the χ(2) values of noble metals are very small, this gives rise to only modest non-linear effects. In order to maximize the NLO efficiency, a better approach would be to combine the nanoparticles with a different material with an already strong NLO coefficient, as was envisioned by Pendry et al., J. IEEE Transactions on Microwave Theory and Techniques 1999, 47, (11), 2075-2084. Previous studies have used this approach to achieve high third-order NLO susceptibilities, where simple mixing of the two components is sufficient to obtain the enhancement. It is more difficult to implement this idea for second order NLO effects, since the material is required to lack global inversion symmetry, which therefore means that a random mixture will have χ(2)=0.
II. Background Detail
II.A. Second Order Nonlinear Optics
In order to possess nonzero second order nonlinear optical susceptibilities, a material must lack a center of inversion at the macroscopic level. The macroscopic second order susceptibility, χ(2) governs the nonlinear polarization P of the medium at frequency ω3 in response to (optical) electric fields E at frequencies ω1 and ω2 throughPiω3=χijk(2)(−ω3;ω1,ω2)Ejω1Ekω2  (1)in which the subscripts refer to the directions of polarization of the fields. Second order nonlinear effects governed by Eq. 1 include second harmonic generation (where (ω1=ω2=ω, so that radiation ω3=2ω is generated at twice the frequency of the incident radiation) and the electro-optic (EO) effect (where ω1=˜0, so that ω2=ω3=ω). In the EO effect, the static field creates a change in the index of refraction at frequency ω. An example of an important χ(2) application is the Mach-Zehnder waveguide modulator in which light is coupled from an optical fiber into the waveguide from the left, and the branch separates the signal into two beams of equal intensity. Due to the electro-optic effect, the refractive index of one arm changes in response to the application of a dc or ac voltage. This changes the optical path length of that arm and modulates the optical signal at the output end because of interference with the light from the other arm. The voltage required to vary the output from maximum value to zero is called the halfwave voltage (Vπ) and is related to the electro-optic coefficient r33 byVπ=(dλ)/(Ln3r33)  (2)in which d is the waveguide thickness, λ is the wavelength, L is the waveguide arm length, n is the refractive index, and r33 is proportional to χ(2). The most common second order NLO materials are ferroelectric, inorganic crystals such as potassium dihydrogen phosphate (KDP), beta-barium borate (BBO), and lithium niobate, this last having a χ(2).=200×10−9 esu and electro-optic coefficient r33=30 pm/volt. Growth of such high-quality inorganic crystals, however, is difficult, time-consuming, and expensive.
II.B. Organic NLO Materials
The χ(2) of an ISAM film is related to the molecular hyperpolarizability (β), chromophore number density (N), dipole tilt angle relative to the film normal vector (ψ), and a local field factor effect (F) by:χ(2)=NFβ<cos3 ψ>  (3)Rational design thus involves choosing a chromophore with a suitably high β and incorporating it into a film with high density N and low tilt angle ψ. As a result of the multitude of frequency conversion, optical modulation, and optical switching applications that stem from the χ(2) second order susceptibility, several methods for creating noncentrosymmetric materials incorporating organic molecules with large β molecular susceptibilities have been developed. These include electric field poled polymers, Langmuir-Blodgett (L-B) films, and covalent self-assembled monolayer structures.
There has been extensive effort for the past two decades in the area of poled polymers, which consist of a glassy polymeric matrix that contains an NLO chromophore either as a guest dopant or as a covalently bonded substituent. Polar order is achieved by orienting the chromophores with an electric field when the material is above the glass transition temperature. The exceptional potential of organic electro-optic materials is illustrated by the demonstrations in poled polymers of full optical modulation at <1.0 V and >150 GHz. While the eventual randomization of the orientation back to the isotropic state has proven a challenging problem, advances continue to be made through use of higher Tg hosts, covalent attachment of the chromophore to the polymer, cross-linked polymers, and dendrimeric structures. Recent materials have shown stability for >1000 hours (42 days) at 85° C. Noncentrosymmetric L-B films with χ(2) values as large as 760×10−9 esu have been fabricated, ˜4 times larger than that of LiNbO3. However, L-B films tend to possess poor mechanical and thermal stabilities owing to the relatively weak van der Waals interactions between layers. Alternatively, several variations have been developed for growth of polar, self-assembled multilayers using siloxane chemistry, but they suffer from long deposition times. A recent procedure can deposit one monolayer in 40 min with χ(2)=430×10−9 esu, clearly an improvement over early approaches, but still impractical for most applications.
Ionic self-assembled multilayer (ISAM) films are a novel class of materials that allows detailed structural and thickness control at the molecular level, combined with ease of manufacturing and low cost. The ISAM method simply involves the alternate dipping of a charged substrate into an aqueous solution of a polycation and an aqueous solution of a polyanion at room temperature. High vacuum, high temperature, organic solvents, or clean-room facilities are not required. The film growth requires that the materials for each successive layer possess multiple charges so that the surface charge on the substrate can be reversed (e.g., from positive to negative) as each layer is adsorbed. The dipping can be repeated to produce a film with as many bilayers as desired. ISAM films are highly robust, rapidly deposited (typically complete in ˜1 minute), and applicable to a vast array of materials that enables wide tuning of refractive index and total thickness. We and others have demonstrated that ISAM films can be fabricated into noncentrosymmetric structures for χ(2) applications. Using commercial ionic polymeric dyes and an NLO-inactive polycation, we have produced ISAM thin films with a noncentrosymmetric arrangement of NLO chromophores that yields χ(2) values comparable to that of quartz, ˜1×10−9 esu, with exceptional temporal and thermal stability. The films have exhibited no measurable decay of χ(2) over a period of more than nine years at 25° C. and more than 18 hours at 150° C.
II.C. Plasmonics
Surface plasmon polaritons (SPPs) are electromagnetic modes that travel along the interface between a metal and a dielectric. The electric field of an SPP has its maximum value at this interface and decays exponentially into the dielectric over a distance comparable to the wavelength (λ), and into the metal over a distance comparable to the skin depth. Just as an electric circuit or a microwave cavity often has one or more resonances, a metallic nanoparticle generally possesses one or more plasmonic resonances, known as localized surface plasmon resonances (LSPRs). The number and frequencies of the LSPRs depend sensitively on the geometry of the nanoparticles along with any coupling to nearby structures. For instance, small spherical nanoparticles have an LSPR that is roughly independent of size, but shifts towards the red if the particle is elongated into a rod, decorated with appendages, or transformed into a cage structure. In this manner, LSPRs of any frequency in the visible, infrared, or below can be achieved, which is important for tuning LSPRs for use at desired wavelengths.
In a manner analogous to the operation of radio antennas, metal nanostructures can concentrate electromagnetic energy into a SPP or LSPR. The ratio of the maximum electric field in the plasmon mode to the field of the free-space wave is referred to as the enhancement factor (g), and it can take on very large values. Even in the simple case of an SPP excited in a thin metal film, g can reach values of 100-200. More interestingly, g˜104 has been measured in nanoparticle arrays, fabricated using a technique known as nanosphere lithography. In nanoparticles dimers, g is predicted to reach values as large as 105. Finally, the largest g ever observed, although controversial, is a colossal 107 or larger, seen in nanoparticle clusters.
This kind of concentration of the electromagnetic field is today used routinely in surface-enhanced Raman spectroscopy (SERS), making Raman measurements of a very small amount of material possible. Even though the mechanism behind Raman scattering is very different from that of second order nonlinear effects, they are alike in that the efficiency in both cases scales as g2. Therefore, plasmonics could become as important to nonlinear optics as it already is to Raman spectroscopy. Such a development has already been anticipated by Pendry et al., who have shown theoretically that if resonant metal structures, smaller than the wavelength λ in size, are filled with a nonlinear material and arranged properly in a transparent medium, the result is a material with a substantially larger nonlinear coefficient than that of the underlying nonlinear filler. While there is a fair amount of ongoing work aimed at exploiting plasmonics for third-order nonlinear applications, most work on second order effects has been limited to investigating the enhancement of SHG from the metal structures themselves, which is inherently a weak effect.
II.D. Metallic Nanoparticles
The surface charge of gold and silver, which are the metals generally used in plasmonics, can be easily controlled in an aqueous solution to be either negative or positive. Thus, ISAM films assemble readily on such surfaces, precisely at the location of the greatest plasmonic field enhancement, and most of the benefit can generally be gained by depositing no more than a few dozen ISAM bilayers. This combination of surface deposition and surface field enhancement makes the marriage of plasmonics with ISAM films potentially extremely fruitful. There are several important design characteristics for metal nanoparticles in organic NLO materials:
The particle LSPR needs to be tunable in the wavelength range of ˜1000-1600 nm.
The nanoparticles should be <200 nm in size so that they can be arranged to minimize light scattering from the materials.
The particle size distribution needs to be as narrow as possible in order to tune the surface plasmon at a particular wavelength and permit effective assembly of particle clusters in layer-by-layer assembled films.
While several methods exist for predicting surface plasmon effects of metal particles as a function of composition, size, shape, and dielectric properties of the particles surroundings, experimental studies still provide the main guidance for designing metal nanoparticles for surface plasmon applications. In particular, two methods—chemical reduction of soluble gold and silver salts in the presence of shape-selecting capping agents and photochemical assembly of silver particles into nanoprisms—have been used to synthesize particles with surface plasmons in the wavelength range 900-1600 nm. Chemical reduction methods include reducing AgNO3 with ethylene glycol in the presence of the nonionic polymer polyvinylpyrollidone (PVP) which acts as a capping agent and as a steric stabilizer. By controlling reagent concentrations, specific shapes can be selected such as wires, nanoprisms, and nanocubes. Theses nanoparticles can be subsequently converted into hollow gold/silver structures. In particular, gold/silver nanocages have been made with dipolar surface plasmon peaks that could be tuned to wavelengths up to 1200 nm. In related work, gold nanoprisms have been produced by the reduction of chloroauric acid by salicylic acid to form gold nanoprisms with edge lengths in the 100-200 nm range and with surface plasmon peaks that range from 1000-1300 nm These nanoprisms were negatively charged and incorporated into ISAM films although NLO effects were not probed. In a variation on this approach, gold nanoprisms were produced by seeding preformed spherical gold particles in chloroauric acid with various reagents including ascorbic acid as a capping agent and cetyltrimethylammonium bromide. High yields of gold nanoprisms resulted with edge lengths in the 50-200 nm range and surface plasmon peaks in the 800-1400 nm range. The seeded growth has also been used with silver particles in a photochemical process in which silver nanospheres made by reduction of AgNO3 using sodium citrate were assembled into silver nanoprisms using controlled irradiation at multiple wavelengths in the range 450-750 nm to select for particles with surface plasmons at wavelengths 800-1500 nm. It has been hypothesized that the controlled irradiation affects the charge distribution on the particle's surface which directs assembly into nanoprisms although the precise mechanism is still not understood.
Metallic nanoparticles can also be fabricated lithographically as well as through chemical synthesis. This can be done through standard lithographic techniques such as optical lithography or electron-beam lithography, or through newer techniques such as nanosphere lithography (NSL). In standard lithography, a substrate is coated with a uniform layer of a resist, which is then lithographically patterned and developed, exposing areas of the substrate where the nanoparticles are meant to be fabricated. Metal is then vacuum deposited onto the substrate, after which the resist is dissolved in an appropriate solvent, which causes the metal deposited onto the resist to lift off from the sample, leaving only the nanoparticles behind. In NSL, the resist is replace with a close packed layer of nanospheres, and metal is vacuum deposited directly onto these sphere. The spheres are then dissolved, leaving a regular array of nanoparticles on the surface.
In summary, several techniques demonstrate facile methods for the synthesis of gold and silver nanoparticles with surface plasmons in the wavelength range of interest and with surface chemistries that are amenable for ready incorporation into organic ISAM films.
However, the prior art detailed above does not provide what is needed, namely, a low cost material having 2nd order non-linear optical (NLO) susceptibilities (χ(2)) that are dramatically larger than those in conventional materials.